A fuel cell stack of the initially named kind is for example known from U.S. Pat. No. 5,441,821. There fuel (hydrogen) is supplied to the anode inlet of a fuel cell stack and hydrogen which is not consumed when flowing through the anode side flow fields of the fuel cell stack is passed through a collecting container for liquid water and is supplied to the anode inlet again together with fresh water. The arrangement is contrived such that the fresh hydrogen is introduced into an injector pump and produces there a suction action which sucks in the returned hydrogen and moves it along together with the fresh hydrogen emerging from the injector pump and feeds it anew to the anode inlet of the fuel cell stack. There it is stated that the proportion of hydrogen which is returned (which contains water vapor) can be mixed with the fresh hydrogen in order to humidify the inflowing fresh hydrogen and to hereby avoid the necessity of a separate humidifying device. It is generally known that so-called PEM fuel cells have to be operated with hydrogen with a certain degree of humidity in order to achieve a high degree of efficiency and to keep the membranes present in the fuel cells moist and to hereby avoid damage with could occur with membranes that are not sufficiently moist.
U.S. Pat. No. 5,441,821 also refers to U.S. Pat. No. 3,462,308 issued Aug. 19, 1969 to Winters which shows a similar system but with an additional discharge valve in the hydrogen return loop which is controlled in order to ensure the correct conditions at the injector pump.
It is moreover known that with a fuel cell stack with return of the hydrogen the proportion of nitrogen and water in the anode circuit rises gradually and leads to a deterioration of the efficiency. In order to counteract this it is known either to continuously discharge a part of the gases that are flowing or to discontinuously open a discharge valve in order to flush the anode side flow circuit from time to time with fresh hydrogen and to keep the efficiency at a high level in this way. Through this manner of operating, which is termed “burp operation” it is possible to significantly stabilize the performance of the fuel cell stack.
Problematic with a manner of operation of this kind is, above all with a compact construction of a fuel cell stack, that a relatively high pressure drop occurs across the fuel cell stack at the anode side. This leads to a situation in which the power requirement for the circulation pump that is used increases. This increase of the power requirement of the circulation pump has on the one hand a negative influence on the overall efficiency of the system and leads on the other hand to a temperature increase of the recirculant after the pump. In order to counteract this increase it was previously necessary to install an additional cooler between the recirculation pump and the fuel cell stack.